(1). Description of the Related Art
Similar methods applicable to monitoring temperature rises, though not for frequency converter supply, are known from GB Patent Application 2151862 and EP Patent Application 350 507, for instance.
(2). Field of the Invention
Electromechanical transmission generates power losses in a squirrel cage induction motor and the losses generated appear as temperature rises of the motor. A magnetizing current to be fed to the motor already causes a temperature rise and upon adding the load the motor warms up more. Part of the heat generated can be caused to flow out of the motor by means of cooling. The outflow of the heat depends on the cooling technique of the motor. The net temperature rise of the motor (temperature rise caused by power losses minus heat outflow) determines the maximum allowed loading capacity. The more the motor is cooled, the more power losses may be allowed for it. The motors are provided with temperature limits, which should not be exceeded. The limits are generally informed both as maximum temperature rise of the insulation of the winding and as maximum temperature rise of the whole motor, whereby the endurance of bearings, for instance, is considered. If the motor exceeds the allowed temperature, the result is either a shortened service life of the motor or, in the worst case, a damaged motor. The limits of the temperature rise of the motor thus restrict the loading capacity. This invention relates to an examination of a temperature rise at the generally most sensible point of the motor, the stator winding. Motor identification plates disclose how much the motor can be loaded by sine wave feed at rated frequency, but the situation becomes more complicated when the supply frequency is changed.
Frequency converter supply causes more power losses in a squirrel cage induction motor than sine wave feed, due to which a motor supplied by frequency converter has a lower loading capacity than a motor having sine wave feed. Additionally, the cooling of the motor causes problems with loading capacity, when a frequency converter is used. The cooling capacity of air-cooled rib-cooled motors changes as a function of frequency, which also causes a change in the maximum loading capacity of the motor as a function of frequency. The maximum loading capacity of especially self-ventilated motors decreases considerably at low speeds when the ventilation capacity is weakening.
Loading capacity is generally informed in relation to rated moment. FIG. 1 of the attached drawing shows a typical moment ratio curve of the loading capacity of a self-ventilated motor when a frequency converter is used, i.e. moment/rated moment in relation to the frequency. The first part (0 . . . 45 Hz) of the curve considers the effect caused by weakening ventilation capacity and the latter part (50 . . . 100 Hz) of the curve considers the effect caused by a field weakening on load moment.
The curve of FIG. 1 is a simplification, which does not illustrate the actual loading capacity accurately. A more accurate idea of the effects of one frequency converter on the loading capacity is obtained from practical measurement results shown in FIG. 2 of the drawing.
When measuring the curve of FIG. 2, the operation of a modulator of the frequency converter used is based on a star modulated PWM (Pulse Width Modulation) technique. The winding behaviour of the loading capacity according to FIG. 2 is caused, except by changes in cooling, by the modulator generating an output voltage.
The loading capacity of the motor is changed at points where modulation is changed, being points at which the number of slices/pulses is changed. The slice number signifies the number of slices during a 60.degree. sector. The number of pulses depends, except on the number of slices, also thereon how many indicators are used per slice.